An influenza (flu) shot is a flu vaccine given with a needle, usually in the arm. Seasonal flu shots protect against the three or four influenza viruses that research suggests may be most common during the upcoming season.
Everyone 6 months of age and older should get a flu vaccine every season with rare exceptions.
There are several reasons why someone might get flu symptoms, even after they have been vaccinated against flu.
One reason is that some people can become ill from other respiratory viruses besides flu such as rhinoviruses, which are associated with the common cold, cause symptoms similar to flu, and also spread and cause illness during the flu season. The flu vaccine only protects against flu, not other illnesses.
Another explanation is that it is possible to be exposed to flu viruses, which cause flu, shortly before getting vaccinated or during the two-week period after vaccination that it takes the body to develop immune protection. This exposure may result in a person becoming ill with flu before protection from vaccination takes effect.
A third reason why some people may experience flu symptoms despite getting vaccinated is that they may have been exposed to a flu virus that is very different from the viruses the vaccine is designed to protect against. The ability of a flu vaccine to protect a person depends largely on the similarity or "match" between the viruses selected to make the vaccine and those spreading and causing illness. There are many different flu viruses that spread and cause illness among people. For more information, see Influenza (Flu) Viruses.
The final explanation for experiencing flu symptoms after vaccination is that flu vaccines vary in how well they work and some people who get vaccinated still get sick. When that happens, though vaccination has been shown in several studies to reduce severity of illness in those people who get vaccinated but still get sick.
Read more at: https://www.cdc.gov/flu/prevent/flushot.htm